This invention pertains to a system used to sweep a magnetic field through the surface of the earth. Systems of this type may be used for geophysical exploration, in situ treatment of surface layers, and certain military and naval purposes.
Prior art arrangements employed for these purposes have utilized permanent magnets, or linear conductors and large area loops of a single conductor which have a high amperage electrical current flowing therein. Such conducting arrangements have limited ranges of angles at which their magnetic fields interact with the earth. The permanent magnetic devices of the prior art have limited magnetic strength. It is often times desirable to direct a high intensity magnetic field at an angle to the earth's surface, such as might characterize a geological strata when encountered on the surface. The prior art arrangements having large magnetic fields with useable ranges are difficult to direct and therefore require computations to consider only the portion of the total field directed in the desired direction.
As a further explanation of the background of the invention, an important military and naval application of the invention is providing a countermeasure sweep for a class of ordnance devices which are magnetically actuated so as to endanger all objects within a predetermined effective range including the activating instrumentality. Certain ones of these devices are activated by the change of magnetic flux along a single axis and may be armed by impact force and a timing mechanism. This arrangement facilitates aerial dropping of individual units in territory which is not under the control of the forces emplanting the devices. In such placements the devices bury themselves by impact force and are armed a predetermined time thereafter.
Devices of the aforedescribed type are particularly difficult to sweep when dropped on land or in shallow waters. Under such conditions, the devices become buried upon impact such that the axis of sensitivity lies at a variety of angular orientations with respect to the surface of the earth. Prior art methods and devices are not able to cause a great enough variation in magnetic field along all possible axis orientations to provide an effective sweeping action from a location sufficiently remote from the device to ensure operational safety.